1. Field of the Invention
The present invention relates in general to a planetary gear type transmission which includes composite planetary gears each having integral large and small pinions. More particularly, this invention is concerned with such a planetary gear type transmission which is constructed for easy and efficient assembling without reducing the strength or durability of the small pinions and without increasing its size, and is also concerned with a method of assembling the transmission. The invention is also concerned with a method which permits easy and efficient assembling of such a transmission.
2. Discussion of the Related Art
As a transmission used on a motor vehicle, for example, there is known a transmission of the type in which a plurality of composite planetary gears are arranged around the shaft of a carrier. Each of the composite planetary gears consists of a large pinion and a small pinion which has a smaller diameter than the large pinion and which is integral with the large pinion for rotation therewith. The large pinions of the composite planetary gears mesh with a common first gear, while the small pinions mesh with a common second gear. Examples of this type of planetary gear type transmission are disclosed in U.S. Pat. No. 4,928,227 to Joseph C. Burbs et al., laid-open Publication JP-A-5-332408 of unexamined Japanese Patent Application No. 4-157513, and Japanese Patent Application No. 5-132935. The transmissions disclosed in these publications are adapted to be used as a speed reduction gear for an electric vehicle.
Referring to the cross sectional view of FIG. 1, there is shown the transmission disclosed in Japanese Patent Application No. 5-132935, wherein the transmission is indicated generally at 10. This transmission 10 includes a sun gear 18 fixed at one axial end of a shaft 16 of a power source in the form of an electric motor, so that the sun gear 18 rotates with the motor shaft 16. The transmission 10 further includes a plurality of (e.g., three) composite planetary gears 24 rotatably mounted on respective pinion shafts 21, and a ring gear 30 secured to a housing 28. The pinion shafts 21 are supported by a carrier 20, and the composite planetary gears 24 are arranged around the axis of the carrier 20. The sun gear 18 and the ring gear 30 are coaxial with the carrier 20, and are rotatable about the axis of the carrier 20. Each of the composite planetary gears 24 consists of a large pinion 22, and a small pinion 26 which has a smaller diameter than the large pinion 22 and which is integral with the large pinion 22. The large pinions 22 of the composite planetary gears 24 mesh with a first gear in the form of the sun gear 18, while the small pinions 26 mesh with a second gear in the form of the ring gear 30.
A rotary motion of the motor shaft 16 received by the sun gear 18 is transmitted to the carrier 20 at a predetermined speed ratio "i", and the rotary motion of the carrier 20 is transmitted to a differential gear 32. The speed ratio "i" is defined as the rotating speed of the sun gear 18 divided by the rotating speed of the carrier 20, and is expressed by the following equation (1): EQU i=1+Zp.sub.1 .multidot.Zr/Zs.multidot.Zp.sub.2 ( 1)
where,
Zp.sub.1 : number of teeth of the large pinion 22, PA1 Zr: number of teeth of the ring gear 30, PA1 Zs: number of teeth of the sun gear 18, PA1 Zp.sub.2 : number of teeth of the small pinion 26
Thus, the sun gear 18 and the carrier 20 operate as the input and output members of the transmission 10, respectively.
The differential gear 32 is a double-pinion type planetary gear device which includes a ring gear 34, a carrier 36 and a sun gear 38. The ring gear 34 is adapted to receive power from the carrier 20, and the power received by the ring gear 34 is distributed to the carrier 36 and the sun gear 38, and transmitted to right and left drive wheels of a vehicle through respective axles 40, 42.
In a transmission having composite planetary gears as described above, the diameters of the large and small pinions are determined depending upon a desired speed ratio of the transmission. If the large and small pinions have different numbers of teeth, it is generally difficult and time-consuming to assemble the carrier and the first and second gears, such that the large pinion of each composite planetary gear supported by the carrier meshes with the first gear, while the small pinion of each composite planetary gear meshes with the second gear. Taking the transmission 10 shown in FIG. 1, for example, first and second planetary gears of the three planetary gears 24 are indicated at 24a, 24b in FIG. 5, wherein the large and small pinions 22, 26 of the planetary gears 24 correctly mesh with the sun gear 18 and the ring gear 30. However, the two teeth of the small pinion 26 of the first planetary gear 24a which are next adjacent to the tooth correctly and fully meshing with the appropriate adjacent teeth of the ring gear 30 are not aligned or matched with the center of any tooth of the large pinion 22, with a certain amount of angular phase shift or difference as indicated in FIG. 5. If one of those two teeth of the small pinion 26 of the first planetary gear 24a is brought into correct full meshing engagement with the appropriate adjacent teeth of the ring gear 30, the large pinion 22 of the first planetary gear 24a cannot mesh with the sun gear 18, as indicated by dashed line in FIG. 5, unless the angular position of the sun gear 18 is changed, that is, unless the meshing positions of the other planetary gears including the second gear 24b are changed with respect to the sun gear 18 and ring gear 30. Where the small pinions 26 of the planetary gears 24 are engaged with the ring gear 30 before the large pinions 22 are engaged with the sun gear 18, the small pinions 26 of all the planetary gears 24 should be initially engaged with the ring gear 30 with predetermined correct angular phases with the ring gear 30, so that the large pinions 22 can be thereafter correctly engaged with the sun gear 18. Where the small pinions 26 of the planetary gears 24 are engaged with the ring gear 30 after the large pinions 22 are engaged with the sun gear 18, the large pinions 22 of all the planetary gears 24 should be initially engaged with the sun gear 18 with predetermined correct angular phases with the sun gear 18, so that the small pinions 26 can be thereafter correctly engaged with the ring gear 30.
If the large and small pinions of each composite planetary gear have the same number of teeth, all teeth of the large pinion have the same angular phase with the corresponding teeth of the small pinion. In this case, the composite planetary gears, sun gear and ring gear can be readily assembled without considering the angular phases or relationships with each other, irrespective of whether the large pinions are engaged with the first gear before or after the small pinions are engaged with the second gear. On the other hand, however, the module of the small pinions should be considerably smaller than that of the large pinions, since the diameter of the small pinions should be considerably smaller than that of the large pinions. Accordingly, the small pinions tend to have reduced strength and durability. In this respect, it is noted that the meshing load acting on the teeth of the small pinions is greater than that of the large pinions, by an amount corresponding to a difference of the diameters of the large and small pinions. Therefore, where the composite planetary gears are subject to a suddenly changing torque as in an automobile transmission, for example, the use of the small pinions having a small module is not desirable. If the diameter of the small pinions is increased to reduce the module to such an extent as to assure sufficient strength and durability of the small pinions, the large pinions and the other gears should have accordingly increased diameters, inherently leading to an increased overall size of the transmission.